Ultraviolet stabilized elastomeric composition



United States Patent O 3,547,880 ULTRAVIOLET STABILIZED ELASTOMERIC COMPOSITION Jackson S. Boyer, Claymont, Del., and Richard D. Cassar,

West Chester, Pa., assignors to Sun Oil Company, Philadelphia, Pa., a corporation of New Jersey No Drawing. Filed Dec. 18, 1967, Ser. No. 691,199 The portion of the term of the patent subsequent to May 12, 1987, has been disclaimed Int. Cl. C08f 45/58; C08c 27/66 US. Cl. 26045.85 25 Claims ABSTRACT or THE DISCLOSURE preparation of said compositions.

CROSS REFERENCE TO RELATED APPLICATIONS The present application is related to copending application Ser. No. 691,153 which discloses hydrocarbon petroleum fractions containing polymethylated muconic acids and/or their esters; and is also related to copending application Ser. No. 691,129 which discloses alpha-olefin polymeric compositions containing polymethylated muconic acids and/or their esters, both applications being filed of even date herewith.

BACKGROUND OF THE INVENTION The present invention relates to the improvement of ultra-violet stability of .elastomers and particularly to modified rubber compositions normally subject to degradation and discoloration resulting from exposure to ultraviolet light.

Specifically, this invention relates to novel elastomer compositions containing specific additives which provide improved ultraviolet stable elastomer compositions and to methods of their preparation.

In the past there has been a considerableproblem concerning the effect of ultraviolet light on elastomers particularly to modified rubber compositions, most noticeably in an oxygen atmosphere containing small amounts of ozone. UV exposure under these conditions usually results in undesirable changes in the elastomer composition. Although there is some controversy as to the exact mechanism involved in the ultraviolet initiated change, most agree that this change represents some form of oxidation of the material affected.

In most applications elastomer compositions normally are subject to exposure to ultraviolet light, either from sunlight or from artificial sources. Ultraviolet degradation in elastomers .is usually noticeable in the form of crazing, cracking, or discoloration of the surfaces of the article. These effects are particularly noticeable on light-colored pigmented compositions, e.g., white sidewalls of tires. UV degradation ultimately contributes to the loss of the attractive appearance of the elastomer as well as a loss in some physical properties thereby providing possible unsafe usage of the elastomeric composition.

It has been reported by H. A. Winkelman in Industrial and Engineering Chemistry, vol. 44, No. 4, pp. 841-850, that deterioration of rubber products outdoors due to the action of sunlight and ozone is a problem of major concern to all manufacturers and consumers of rubber products. Automotive manufacturers each year face losses aggregating into thousands of dollars because of premature cracking and failure of rubber parts.

Recent advances in rubber technology are contributing to the development of long-life rubber compositions particularly suitable for use in automobile and truck tires. Primarily these advances have been made in the development of abrasive-resistant treads which theoretically double the mileage life of a tire. However, although the abrasive-resistance properties of tire treads have been substantially improved, the inherent susceptibility of elastomers to break down resulting from exposure to ultraviolet light remains an ever-present weakness. This weakness presents a primary limiting factor in the life of an automobile or truck tire or any of the many applications for which rubber is used.

Many organic rubber stabilizers such as N-isopropyl- N'-phenyl-p-phenylene diamine, N-phenyl-N'-cyclohexylp-phenylene diamine, 6-ethoxy-1,2-dihydro-2,2,4-trimethyl quinoline, N,N'-di-( l-methylheptyl)-p-phenylene diamine, and diphenyl-p-phenylene diamine homologs are presently being added to rubber compounds prior to molding and vulcanization to inhibit UV degradation. However, some of these additives either provide only limited protection or have staining characteristics which render them undesirable for many commercial applications.

DESCRIPTION OF THE INVENTION It has now been discovered that certain di-, tri,- and tetramethylmuconic acids hereafter referred to as polymethylated muconic acids and their esters when blended with elastomers provide an elastomeric composition having improved resistance to ultraviolet initiated degradation.

It has been discovered that the cis-cis, cis-trans, or trans-trans isomers of u,a'-dimethylmuconic acid, 0M3- dimethylmuconic acid, a,u',fl-trifmethylmuconic acid, a,,B, 3'-trimethylmuconic acid, a,a,;9,B'-tetramethylmuconic acid, or their monoesters wherein one carboxyl group is attached to a C -C hydrocarbyl radical, or their diesters wherein each carboxyl group is attached to a C -C hydrocarbyl radical, and mixtures thereof when added to an elastomer in the quantity of 0.1-10.0 parts additive per parts of elastomer provide an elastomeric composition having improved resistance toultraviolet degradation. The preferred concentration of polymethylated muconic acid or its esters in the elastomer composition is generally in the range of 1.0-5.0 parts by weight per 100 parts of elastomer.

Each of the above-disclosed polymethylated muconic acids in the cis-cis, cis-trans, or trans-trans isomeric form or mixtures thereof is effective for use in the compositions of the present invention. Also, the C -C hydrocarbyl monoesters or diesters of these acids or mixtures thereof are effective for use in the compositions of the present invention.

The C C hydrocarbyl esters included in the present invention are selected from the hydrocarbyl radicals of C C hydrocarbons having acyclic, cyclic, and aromatic structures such as those disclosed in the text Handbook of Hydrocarbons, S. W. Ferris, Academic Press Inc., New York, NY. (1955), pages -249, all of which are incorporated herein by reference. The preferred esters of the present invention are the C C hydrocarbyl monoand diesters of the polymethylated muconic acids disclosed above. Examples of the C C hydrocarbyl groups include methyl, ethyl, butyl, pentyl, hexyl, heptyl, octyl, nonyl, decyl, cyclopentyl, methyl cyclopentyl, dicyclopentyl, cyclohexyl, phenyl, tolyl, xylyl, naphthyl, tetrahydronaphthyl, decahydronaphthyl, as well as the various isomers of each.

The diester of the muconic acid can be a mixed ester. An illustrative example is the cis-cis isomer of oc,oc'-di methylmuconic acid which can be illustrated by the following structural formula:

H CHs wherein R is different from R That is to say, R can be a hydrocarbyl group of C -C and R can be a different hydrocarbyl group of C -C e.g., R equals cyclohexyl (C and R equals eicosyl (C Examples of some of the esters of the polymethylated muconic acids suitable for use in the compositions of the present invention include the cis-cis, cis-trans, and transtrans isomers of the monoand di-methyl esters of a,fl'- dimethylmuconic acid; the monoand di-phenyl esters of a,a,B, 3-tetramethylmuconic acid; the monoand dinaphthyl esters of a,;3,/3-trimethy1muconic acid; the mono and di-5,6-diethylacenaphthyl esters of 0L,ot'-dlII161hylmuconic acid; the monoand di-cyclohexyl ester of u,oc'- dimethylmuconic acid; the monoand di-1,2-dimethylcycloheptyl esters of u,fi'-dimethylmuconic acid; the monoand di-decahydronaphthyl esters of a,a',}8,,8'-tetramethylmuconic acid; the monoand di-1,3-dipropylbenzyl esters of a,a'-dimethylmuconic acid; the monoand di-2,9-dimethyl-4,7-diisobutyldecyl esters of a,a,fl-trimethylmuconic acid; and the monoand di-anthracyl esters of a,fi'- dimethylmuconic acid; monoand di-2,6,l0-trimethyl decyl esters of a,a,;3,;8'-tetramethylmuconic acid; and the nonyl ethyl esters of a,a',fl-tetramethylmuconic acid.

Specifically it has been discovered that the cis-cis, cistrans, or trans-trans isomers of the following compounds are effective ultraviolet stability improvement additives when dispersed in elastomeric compositions. These additive compounds are selected from:

(a) Polymethylated muconic acids selected from the group consisting of a,a'-dimethylmuconic acid, a,;3'-dimethylmuconic acid, a,u',,9-trimethylmuconic acid, 843- trimethylmuconic acid, and a,a,fl,;8-tetramethylmuconic acid,

(b) The hydrocarbyl monoesters of said diacids wherein the hydrocarbyl group contains 1-20 carbon atoms,

(0) Hydrocarbyl diesters of said diacids wherein each hydrocarbyl contains 1-20 carbon atoms, and

(d) Mixtures of any of the above. As noted above, polymethylated muconic acids can exist in three isomeric forms, viz. cis-cis, trans-trans, and cistrans. As an example, the unsaturated diacid, a,a'-dimethylmuconic acid, can be depicted by the following structural formulas:

The preparation of each of these isomeric forms of the a,u'-dimethylmuconic acid has been described in the prior art by Elvidge et 211., J. Chem. Soc., pages 1026- 1033 (1952). These authors show that oxidation of p-xyle- 1101 by means of peracetic acid gave the cis-cis form of the acid. The other isomeric forms were obtained indirectly by conversion of the cis-cis form. Also dimethyl esters of each of the three isomeric forms were prepared by shaking the respective DMMA with ethereal diazo methane.

The cis-cis form of polymethylated muconic acids can also be obtained by biological oxidation of p-xylene utilizing special strains of microorganisms as disclosed in United States application Ser. No. 509,621, filed Nov. 24, 1965, and now U.S. Pat. No. 3,383,289 issued May 14, 1968.

Procedures for recovering esters of the three isomeric forms of methylated muconic acids usable in the compositions of the present invention are also disclosed in United States application Ser. No. 561,736, filed June 30, 1966, and now U.S. Pat. No. 3,440,158 issued Apr. 28, 1969.

By the term elastomer as herein disclosed is meant natural or synthetic polymers which exhibit the property of long range elasticity. By long range elasticity is meant the ability of the polymer to undergo stretching to at least 150 to 200 percent of its original length and to retract very rapidly to virtually its original length when it is released. A full description of elastomers useful in the compositions of the present invention is disclosed in The Encyclopedia of Chemistry, 2nd ed., Reinhold Publ. Co., New York, N.Y. (1966), pages 359-362.

, Included as elastomers are rubbers of either natural or .synthetic origin.

By the term rubber as herein disclosed is meant natural or synthetic rubber that has been modified to increase its useful properties such as elasticity, toughness, resistance to abrasive wear, and others. The modification of the elastomer is accomplished usually by compounding or masticating the rubber with sulfur or other vulcanizing agents. The composition can also contain various other additives such as zinc oxide, carbon black, or other reinforcing pigments, fillers, softeners, extenders and antioxidants. The elastomer and additive composition is thereafter shaped and vulcanized. These compositions are chiefly used in tires, hose, belting, friction materials, containers, electrical insulating, and water proofing materials, and can also be used in combination with textile fabrics and metals as well as other materials.

Examples of elastomers usable in the compositions of the present invention include natural and synthetic rubber compositions which are normally subject to ultraviolet initiated degradation. Natural rubber including Hevea braziliensis latex and synthetic rubber including butyl polymers, such as polybutene-l, polyisobutylene, polybutadiene, as well as styrene-butadiene copolymers (SBR), ethylene-propylene dicyclopentadiene terpolymers, polyisoprene, neoprene (polychloroprene), acrylonitrile-butadiene polymers, among others, are included among the compositions of the present invention.

As a means of illustrating one mode of the present invention, the following evaluations are given. All parts as hereinafter disclosed are given as parts by weight.

EXAMPLE I A sample of raw styrene-butadiene rubber (SBR) containing 23.5 weight percent polymerized styrene and having a Mooney viscosity of 52 was compounded and vulcanized in the following manner:

parts of SBR were banded on a standard rubber mill while maintaining roll temperatures at approximately 73 F. After 2-3 minutes of milling the SBR, 10 parts of zinc oxide, 10 parts of titanium dioxide, and 20 parts of a standard naphthenic rubber processing oil, 2.0 parts sulfur and 1.75 parts benzothiazyl disulfide were blended with the rubber and the whole composition was milled for about thirty minutes toprovide complete dispersion of additives in the rubber. The rubber composition was thereafter sheeted to provide rubber specimens having the dimensions of 6 x 6 x .075 inches. The rubber sheets were subsequently vulcanized at a temperature of 300 F. for

about 45 minutes. The vulcanized sheets were then quenchedwith cold water and dried.

. The vulcanized rubber samples were thereafter evaluated for ultraviolet stability by exposing each sample to ultraviolet light for periods of 24, 48, and 72 hours in accordance with the procedures outlined in ASTM D925-55.

One noticeable effect of exposure of white rubber compositions to ultraviolet light is discoloration of the sample in the form of increased yellowness. The increase 1n intensity of the yellowness of the rubber sample is usually indicative of the increased degradation of the rubber initiated by ultraviolet light. The intensity of yellowness of the rubber samples herein tested were determined on a Photo Volt Corporation Model 610 Reflectometer having a standard blue filter. The results of this evaluation appear in the table under Example I. In the determination the intensity of yellowness is inversely proportional to the reflectance value shown in the Reflectometer. This Example I serves as a reference example illustrating one effect of ultraviolet light on an unstabilized elastomeric composition.

EXAMPLE II A sample of the dimethyl ester of trans-trans mod-dimethylmuconic acid was prepared as follows:

A solution of 150 g. (0.88 mol.) of cis-cis dimethylmuiconic acid obtained by biological oxidation of p-xylene and dissolved in 1 liter of 6 N NaOH is refluxed for 24 hours, diluted with 5 volumes of distilled water, and brought to pH 3 with 12 N HCl. The solid which separates at this point is filtered oil and dried; it contains about 80% of the trans-trans acid, with a lesser amount of the cis-trans acid and a minor amount of the cis-cis acid.

Thecrude product thus obtained is esterified by refluxing it (140 g.) in 1 liter of methanol containing 1 cc. of concentrated H 80 until solution occurs. Cooling the reaction mixture yields the dimethyl ester of the trans-trans acid in substantially pure form (M.P. 102104 C.).

A rubber composition identical to that of Example 1 was compounded and cured in the identical manner as Example I with the exception that 0.1 part of the dimethyl ester of trans-trans a,a'-dimethylmuconic acid prepared as disclosed above was added to the rubber composition during the compounding step. This rubber sample was evaluated. in the identical manner as the rubber compositions of Example I. The results of this evaluation appear in the table under Example II.

EXAMPLE III Example I was repeated with the exception that 0.5 part of the dimethyl ester of trans-trans e o/dimethyl muconic acid was added to the rubber composition during the compounding step. The results of this evaluation appear in the table under Example III.

EXAMPLE IV Example II was repeated with the exception that 1.0 part of the'dimethyl ester of trans-trans c d-dimethylmuconicacid was added to the rubber composition during the compounding step. The results of this evaluation appear in the table under Example IV.

. EXAMPLE V EXAMPLE VI Example II was repeated with the exception that 5.0 parts of the dimethyl ester of trans-trans a,a'-dimethylmuconic acid was added to the rubber composition during the compounding step. The results of this evaluation appear in the Table under Example VI.

6 EXAMPLE VII Example II was repeated with the exception that 8.0 parts of the dimethyl ester of trans-trans a,a'-dimethylmuconic acid was added to the rubber composition during the compounding step. The results of this evaluation appear in the table under Example VII.

TAB LE Reflectance values after- 24 hrs. 48 hrs. 72 hrs.

Parts of No U.V. U.V. U.V. U.V.

Example additive exposure exposure exposure exposure None 91.0 37. 5 19. 0 18.5

An ultraviolet stability evaluation of unstabilized SBR is represented by the data given in the Table under Example I. The data given in the table under Examples II-VII when compared with Example I illustrate the substantial improvement achieved in ultraviolet stability of SBR by addition and dispersion of small quantities of the dimethyl ester of trans-trans u,ot'-dimethylmuconic acid in the rubber composition prior to vulcanization of the composition.

As can be seen from the results of Example I given in the Table, prolonged exposure to ultraviolet light reduces the reflectance value of unstabilized rubber indicating an increase in yellowness of the rubber. The more intense the yellowness of the rubber, the greater the degree of ultraviolet initiated degradation that has taken place in the rubber.

The rubber samples containing incremental quantities of one of the ultraviolet stabilizer compositions of the present invention exhibit a substantial decrease in measured yellowness intensity as compared to the same rubber unstabilized, and therefore demonstrate their improved resistance to ultraviolet initiated hydradation.

The reflection values for each sample given above was measured on a Photovolt Corporation Model 610 Reflectometer having a tungsten lamp and a blue filter lense. The test comprises subjecting the sample to be tested to the photoelectric cell of the Refleetometer and reading the amount of reflectance for that sample directly from the scale of the galvanometer of the instrument.

In the above-given series of evaluations the photoelectric cell was adjusted for yellowness determinations in the following manner:

(a) A grey ceramic tristimulus color evaluation plaque having a blue color value of 48.0, a green color value of 45.0 and an amber color value of 44.0 was placed on the face of the photoelectric cell and the galvanometer Was adjusted to a value of 50, and (b) A white ceramic tristimulus color evaluation reference plaque having a blue color value of 75.5, a green color value of 74.5 and an amber color value of 74.0 was placed on the face of the photoelectric cell and the galvanometer scale was adjusted to a reading of 'Each sample tested was thereafter placed on the face of the photoelectric cell and the reflectance value for the sample was taken as a direct reading from the galvanometer scale. The values achieved by this method are relative values and provide a bases for comparison of intensity of yellowness of the samples. Another procedure which is equally useful in determining the yellowness of white rubber is disclosed in ASTM D1925-63T.

The present examples illustrate the evaluations on white pigmented rubber. However, any rubber containing any form of pigmentation filler, extenders or other additives normally associated with vulcanized rubber including carbon black can be used in the compositions of the present invention and achieve improved resistance to ultraviolet degradation.

Elastomeric compositions other than SBR can be combined with the ultraviolet stability improving additives disclosed hereinabove within the scope of the present invention to provide results analogous to those disclosed in said samples. Such compositions are included within the scope of the compositions of this invention.

Ultraviolet stability improving additives other than the dimethyl ester of trans-trans a,ot-dimethylmuconic acid set forth in the examples given above can be substituted therefor in any of the elastomers disclosed and provide analogous results. Included among these additives are the cis cis, cis-trans, and trans-trans isomers of any of the polymethylated muconic acids disclosed above as well as these isomers of the mono esters of each of these acids wherein the ester is a C -C hydrocarbyl group and also the diesters of each of these acids wherein each ester is a C -C hydrocarbyl group. Any combination of the above additives with any of the disclosed elastomers provide results analogous to those disclosed in the examples disclosed herein.

Elastomer compositions of any of the above-disclosed polymers in the aqueous latex emulsion state or as the uncured raw rubber precipitated from latex emulsions are also suitable as compositions which can be improved with respect to ultraviolet stability by the methods hereinabove disclosed. Raw rubber lattices containing an ultraviolet stability improving quantity of the polymethylated muconic acids and muconates hereinabove disclosed are included in the scope of the present invention.

We claim:

1. A composition having improved ultraviolet stability comprising an elastomer selected from the group consisting of natural rubber and synthetic rubber derived from an ethylenically unsaturated monomer and containing an ultraviolet stability improving quantity of a material selected from the group consisting of:

(a) polymethylated muconic acids selected from ot,ot'-

dimethylmuconic acid, a,fl'-dimethylmuconic acid, a,a',B-trimethylmuconic acid, a,B,/3'-trimethylmuconic acid, and a,a', 3,}8-tetrametylmuconic acid,

(b) hydrocarbyl monoesters of said muconic acids wherein the hydrocarbyl group contains 1-20 carbon atoms,

(c) hydrocarbyl diesters of said muconic acids wherein the hydrocarbyl groups each contain 1-20 carbon atoms, and

(d) mixtures thereof.

2. A composition according to claim 1 wherein the polymethylated muconic acids are the cis-cis isomers.

3. A composition according to claim 1 wherein the polymethylated muconic acids are the cis-trans isomers.

4. A composition according to claim 1 wherein the polymethylated muconic acids are the trans-trans isomers.

5. A composition according to claim 1 wherein the quantity of ultraviolet stability improving material is in the range of 0.1-10.0 parts by weight per 100 parts of elastomer.

6. A composition according to claim 2 wherein the quantity of ultraviolet stability improving material is in the range of 0.1-l0.0 parts by weight per 100 parts of elastomer.

7. A composition according to claim 3 wherein the quantity of ultraviolet stability improving material is in the range of 0.1-10.0 parts by weight per 100* parts of elastomer.

8. A composition according to claim 4 wherein the quantity of ultraviolet stability improving material is in the range of 0.1-10.0 parts by weight per 100 parts of elastomer.

9. A composition according to claim 1 wherein the elastomer .is styrene-butadiene rubber.

10. A composition according to claim 5 wherein the elastomer is styrene-butadiene rubber.

11. A composition according to claim 1 wherein the elastomer is ethylene-propylene-dicyclopentadiene rubber.

12. A composition according to claim 5 wherein the elastomer is ethylene-propylene-dicyclopentadiene rubber.

.13. A composition according to claim 1 wherein the ultraviolet stability improving material is the dimethyl ester of a,a-dimethylmuconic acid.

14. A composition according to claim 4 wherein the ultraviolet stability improving material is the dimethyl ester of a,a'-dimethylmuconic acid.

15. A composition according to claim 5 wherein the ultraviolet stability improving material is the dimethyl ester of a,a'-dimethy1muconic acid.

16. A composition according to claim 6 wherein the ultraviolet stability improving material is the dimethyl ester of a,a-dimethylmuconic acid.

17. A composition according to claim 9 wherein the ultraviolet stability improving material is the dimethyl ester of a,u-dimethylmuconic acid.

18. A composition according to claim 1 wherein the elastomer is styrene-butadiene rubber, the ultraviolet stability improving material is the dimethyl ester of transtrans a,a-dimethylmuconic acid and the quantity of said material in the composition is in the range of 0.1-10.0 parts by weight per parts of said rubber.

19. A composition according to claim 1 wherein the quantity of ultraviolet stability improving material is in the range of 0.1-5.0 parts by weight per 100 parts of elastomer.

20. A composition according to claim 2 wherein the quantity of ultraviolet stability improving material is in the range of 0.1-5.0 parts by weight per 100 parts of elastomer.

21. A composition according to claim 3 wherein the quantity of ultraviolet stability improving material is in the range of 0.15.0 parts by weight per 100 parts of elastomer.

22. A composition according to claim 4 wherein the quantity of ultraviolet stability improving material is in the range of 0.1-5.0 parts by weight per 100 parts of elastomer.

23. A composition according to claim 9 wherein the quantity of ultraviolet stability improving material is in the range of 0.1-5.0 parts by weight per 100 parts of elastomer.

24. A composition according to claim 11 wherein the quantity of ultraviolet stability improving material is in the range of 0.1-5.0 parts by weight per 100 parts of elastomer.

25. A composition according to claim 13 wherein the quantity of ultraviolet stability improving material is in the range of 0.1-5.0 parts by Weight per 100 parts of elastomer.

References Cited UNITED STATES PATENTS 2,897,176 7/1959 Rocky et a1. 260-4557 3,115,465 12/ 1963 Orloff et al. 260-457 3,248,248 4/1966 Coran 260-4585 3,256,238 6/ 1966 Anognostopoulos 260-4585 3,256,312 6/1966 Strobel 260-4585 3,280,069 10/ 1966 Knapp 2160-45785 3,395,111 7/1968 Mazzolini et al. 26045.75

FOREIGN PATENTS 637,808 3/1962 Canada 260-45.75 940,028 10/ 1963 Great Britain 260-4575 DONALD E. CZAJA, Primaly Examiner 0 VERONICA P. HOKE, Assistant Examiner US. Cl. X.R. 

